The U.S. State Department has ordered all nonessential staff out of Yemen and told U.S. citizens to leave the country "immediately" amid heightened security in the region against a possible terrorist attack.

The warning comes after the closure of 19 U.S. diplomatic missions across the Middle East and Africa.

That action was apparently prompted by reports of intercepted messages between Al-Qaeda leader Ayman Al-Zawahri and Nasser al-Wahishi, the head of the Yemen-based Al-Qaeda in the Arabian Peninsula (AQAP).

The intercepts apparently indicate an attack was being planned in the region.

Britain said it had withdrawn all staff from its embassy in Yemen's capital, Sanaa.

The State Department warning comes just hours after reports about a suspected U.S. drone strike in Yemen. Unnamed local security officials said the strike on August 6 killed four alleged Al-Qaeda operatives in the eastern province of Marib.

The officials say they believe at least two of the dead were senior members of AQAP, which the United States considers the most dangerous Al-Qaeda affiliate.

Britain, France, Germany, and Norway have also closed some of their diplomatic missions in the region.

Michael McCaul, the chairman of the House Homeland Security Committee has said that the United States was facing "probably one of the most specific and credible threats I've seen, perhaps, since 9/11."

U.S. State Department deputy spokeswoman Marie Harf said on August 5 that the situation called for exceptional measures.

"Overall, what we are doing is taking precautionary steps out of an abundance of caution to protect our people, and our facilities, and visitors to those facilities overseas," the spokeswoman said.

U.S. officials were not specific as to the nature of the threat and some admitted the exact target was unknown.

The suspension of operations at the U.S. diplomatic missions is due to last through the rest of Ramadan, the Muslim holy month that ends this weekend.